Coil shaved metal fibers are known in the art from U.S. Pat. No. 4,930,199. Coil shaved fibers comprising several layers of different metals or metal alloys are known from WO2005/028135A1. Coil shaved metal fibers typically have a substantially rectangular cross section, due to the process of coil shaving. This is different from e.g. bundle drawn metal fibers, which have usually a pentagonal or hexagonal cross section. The coil shaved metal fibers also differ from other machined metal fibers such as steel wool. Steel wool usually has a rather triangular cross section.
The presently known coil shaved fibers have several disadvantages, which disadvantages are linked to process limitations of the presently known processes for shaving metal fibers from a coiled metal foil.
The presently known coil shaved metal fibers may only be provided at relatively high equivalent diameters. This is because the equivalent diameter is dependent on the thickness of the foil used on the one hand, and the cutting rate, i.e. the advancement of the cutting tool per rotation of the coil provided on the rotating spindle in axial direction of the spindle, on the other hand. Although some metal foils of 10 μm thickness are known, the provision of machineable foils with thicknesses of less than 35 μm is difficult if not impossible for a considerable number of metal alloys. On the other hand, it seems to be difficult if not impossible to shave at a cutting rate of less than about 10 μm. At present, the foil thickness is chosen larger than the cutting rate. When the foil is too thin, or the cutting rate is too small, the shaving operation is subjected to numerous process interruptions, i.e. broken bundles of metal fibers.
It is believed that these ruptures of coil shaved fibers are due to upsetting of the fiber surface due to the shaving operation itself.
It is found that coil shaved metal fibers are rather uneven, especially along the sides which have been in contact with the cutting tool during shaving. These are the sides which have a length substantially equal to the thickness of the foil used. It is believed that the upset causing such a rough surface creates weak spots at the mantle surface of the fibers along the fiber length. As the foil thickness is usually chosen larger than the cutting rate, the rough surface is present along at least more than 50% of the fiber surface.
In spite of their relatively large equivalent diameter, coil shaved metal fibers as presently known are rather fragile, which is also believed to be due to the upset at the fiber surface and the creation of numerous weak spots. The force at rupture in function of the fineness of the fiber (tex) (also known as specific stress and expressed in N/tex) or in function of the surface area of a fiber radial cross sections, thus in function of equivalent diameter, (also known as stress and expressed in N/μm2) is very low as compared to usual comparable values of metal wires made from the same metal material.
Presently known coil shaved fibers also show a relatively large deviation of the equivalent diameter of the metal fibers in a bundle of coil shaved metal fibers. This large deviation renders the bundles of coil shaved metal fibers not useful for particular applications, and necessitates the use of more expensive bundle drawn metal fibers with this equivalent diameter.